We report the experimental finding of n=1 helical cores (HCs) accompanied by saturated m/n=2/1 tearing modes (TMs) with low mode frequencies in JT-60U. The HCs accompanied by TMs were observed after an increase in the mode amplitude and a decrease in the mode frequency of m/n=2/1 precursors with tearing parity. The decreased mode frequency is typically lower than 20 Hz. With various diagnostics, the coupling of n=1 HCs and m/n=2/ 1 TMs has been clearly observed. Because the coherent oscillations in the ion temperature are observed in both the core region and the edge region, the flux surfaces including the m/n=2/1 magnetic island appear to have m=1 helical deformation. It has also been suggested that the m/n=2/1 TM and the HC rotate in the electron diamagnetic direction keeping f m/n=1/1(HC) =2f m/n=2/1(TM) in several plasmas. Here, f m/n=1/1(HC) is the mode frequency of HCs and f m/n=2/1(TM) is the mode frequency of TMs. In addition, the core seems to be shifted to the high-field side when the O-points of the m/n=2/1 magnetic island line up in the midplane, which is confirmed by reconstructions of magnetohydrodynamic equilibria with motional Stark effect measurement and the MEUDAS code. Our observation of m/n=2/1 TMs having HCs contributes to the understanding of the excitation mechanism of HCs in tokamak plasmas.

To realize a compact spherical tokamak (ST) reactor, operation without the central solenoid (CS) must be demonstrated. In particular plasma current (I p) ramp-up from zero to a level required for fusion burn is crucial. Plasma initiation and I p ramp-up in ST by waves in the lower-hybrid (LH) frequency range were demonstrated for the first time on TST-2. A combline antenna was used to inject RF power of ~ 100 kW at 200 MHz. Formation of a low current (~ 1kA, mainly driven by pressure gradient) ST configuration can be accomplished by waves over a broad frequency range (21 MHz to 8.2 GHz in TST-2), but further I p ramp-up (to ~ 10 kA, mainly driven by RF) is most efficient with uni-directional traveling waves in the LH frequency range. I p ramp-up to 15 kA was achieved with 60 kW of net RF power. Soft X-ray emission in the direction of electron acceleration by RF wave was enhanced more strongly in the co-drive case (acceleration in the direction to increase I p) compared to the counter-drive case. Hard X-ray spectral measurements showed that the photon flux is an order of magnitude higher and the photon temperature is higher in the co-current-drive direction than in the counter-current-drive direction. These observations are consistent with acceleration of electrons by a unidirectional RF wave. The combline antenna excites vertical electric fields which match the polarization of the fast wave (FW). There is evidence that the LH wave (or the slow wave, SW) is excited nonlinearly, based on the frequency spectra measured by magnetic probes in the plasma edge region. The time evolution indicates the tendency of the pump wave to weaken when the sideband waves intensify. It is expected that the effectiveness of current drive would improve if the LH wave could be excited directly by the antenna. Two types of travelingwave LH antennas will be tested on TST-2, a dielectric-loaded waveguide array ("grill") antenna, and an array of capacitively coupled elements with the electric field polarized in the toroidal direction. During initial operation of the grill antenna, wavenumber components were measured by an array of magnetic probes. Results were qualitatively consistent with expectations based on dispersion relations for the FW and the SW.

Spherical tokamaks (STs) have the advantage of superior stability at high beta, but to realize a compact fusion reactor, the central solenoid (CS) must be eliminated. The plasma current could be maintained mainly by the self-driven current during the steady-state burning phase, but there is no established method of non-inductive current ramp-up from zero to a high enough level required for fusion burn. The lower-hybrid wave (LHW) is commonly used in tokamaks, but this technique is believed to be unusable in ST plasmas with very high dielectric constant. Wave excitation, propagation and damping were evaluated by numerical modeling for the TST-2 spherical tokamak (R = 0.68 m, a = 0.25 m, B = 0.3 T, I = 100 kA). Wave excitation is calculated by the RF antenna simulation tool based on the finite element method (COMSOL). The plasma is modeled as a medium with cold plasma dielectric response and artificially enhanced loss. Excitation of a traveling fast wave (FW) by the TST-2 combline antenna was confirmed by COMSOL calculation. However, the FW must be mode converted to the LHW to achieve efficient current drive. The TORLH full-wave solver, which can treat diffraction effects properly, was applied to the TST-2 LH current drive experiment. It is shown that core current drive by LHW is possible in the low density, low current plasma formed by ECH. It is important to keep the density low during current ramp-up, and the wavenumber must be reduced as the current increases in order to maintain core current drive. The results of these calculations and the initial experiment on TST-2 will be used to design an optimized LHW antenna with appropriate polarization and wavenumber spectrum controllability for the current ramp-up experiment. Taking a conservative value for the current drive figure of merit, the steadystate driven current by 200 kW of RF power is estimated to be 150 kA, which should be adequate for evaluating the usefulness of this technique up to a current level of 100 kA in TST-2. This technique should be usable during the low-density, low-current, non-burning start-up phase of a reactor to reach a sufficient current level needed for further heating. Initial results from the TST-2 experiment demonstrated that initial current formation and ST equilibrium formation can be achieved by RF power in the LH frequency range. Further current ramp-up by directly excited LHW will be investigated on TST-2. The success of the TST-2 experiment would provide a scientific basis for quantitatively evaluating the required CS capability for a low aspect ratio reactor.

The University of Tokyo Spherical Tokamak (UTST) was constructed to explore the formation of ultrahighbeta spherical tokamak (ST) plasmas using double null plasma merging. The main feature of the UTST is that the poloidal field coils are located outside the vacuum vessel to demonstrate startup in a reactor-relevant situation. Initial operations used partially completed power supplies to investigate the appropriate conditions for plasma merging. The plasma current of the merged ST reached 100 kA when the central solenoid coil was used to assist plasma formation. Merging of two ST plasmas through magnetic reconnection was successfully observed using two-dimensional pickup coil arrays, which directly measure the toroidal and axial magnetic fields inside the UTST vacuum vessel. The resistivity of the current sheet was found to be anomalously high during merging.

The JT-60SA tokamak, being built under the Broader Approach agreement jointly by Europe and Japan, is due to start operation in 2020 and is expected to give substantial contributions to both ITER and DEMO scenario optimisation. A broad set of preparation activities for an efficient start of the experiments on JT-60SA is being carried out, involving elaboration of the Research Plan, advanced modelling in various domains, feasibility and conception studies of diagnostics and other sub-systems in connection with the priorities of the scientific programme, development and validation of operation tools. The logic and coherence of this approach, as well as the most significant results of the main activities undertaken are presented and summarised.